Most velocities are listed as being from 24' barrels. With a .243 (or similar) you can use a rule of thumb of about 30 FPS +/- loss of velocity for every inch less of barrel length. From 24" to 18" would then mean a loss of about 180 FPS or so. Each rifle is different but these numbers should be close enough to give you an idea and to re-calculate trajectory numbers, should you feel that necessary.